There are many applications for highly integrally bonded dissimilar metals. In particular, bonding aluminum to dissimilar metals, particularly copper, is useful in applications requiring high thermal or electrical conductivity, and has been the subject of searches and studies in the industry. However, although copper is an excellent heat and electrical conductor, copper has a high CTE (coefficient of thermal expansion) that does not match the CTE for semiconductor materials.
In the industry, in order to bond a copper heat sink to a DBC (direct bond copper substrate), a thick layer of solder is generally used. Solder, having some plasticity, is employed to absorb the CTE mismatch between copper and a DBC. Although a thick solder layer has proven useful, the high thermal resistance of such a layer presents problems for high power electronic components, and solder may develop cracks with repeated temperature cycles.
U.S. Pat. No. 6,897,123 to Winther (2005) is a representative of a number of prior art patents, which provide methods to bond dissimilar materials having different CTEs. Winther teaches the use of a functionally graded interlayer, which provides a different CTE on each mating face. The material is made using a resin and filler particles. The prior art does not deal effectively with the bonding of dissimilar metals, or the bonding of materials with poor thermal conductivity, such as resins.
Aluminum is a metal that is very difficult to bond to other materials or to itself. Aluminum is highly reactive to oxygen, nitrogen and argon. The aluminum base metal forms a tenacious non-permeable oxide layer upon contact with the atmosphere. This oxide layer protects the base metal from further oxidation. The oxide layer can grow to 5.0 nm in a natural environment, and in a hot environment (300° C.) the oxide layer can grow to 30 nm thick. If the oxide layer is removed, it will immediately reform. The oxide layer is a good dielectric layer and has poor thermal conductivity, and thus prevents another material from bonding with the aluminum base metal. In any application that requires bonding aluminum and copper, the aluminum surface must be free of all oxide particles.
In general there are three methods used to remove aluminum oxide: mechanical, chemical, and plasma. There are also combinations of these three methods. The mechanical method usually involves drawing a stainless steel wire brush rapidly across the bonding surface or machining the surface. Chemical methods to remove aluminum include using acid (nitric acid, HNO3) solutions or alkaline (sodium hydroxide, NaOH) solutions. U.S. Provisional Patent Application No. 62/097,030 teaches the use of an acid consisting of 80% phosphoric acid (H3PO4)+5% acetic acid (CH3COOH)+5% nitric acid (HNO3)+10% water (H2O). Plasma cleaning processes have also been used to successfully remove aluminum oxide. If the aluminum part is attached to a negative pole, a stream of positive ions will bombard the surface, and break up the aluminum oxide layer. The dislodged particles can be removed by a plasma arc. U.S. Pat. No. 4,030,967 to Ingrey, Nentwich, and Poulsen (1977) describes a plasma etching process that removes aluminum oxide using gaseous trihalide in a radical-flow type reactor. However, after the aluminum oxide layer is removed, the aluminum component must be processed quickly because a new oxide layer immediately begins to form as soon as the cleaning process ends.
There are many solid state methods known to bond aluminum to aluminum or to a dissimilar metal. In roll bonding, the surfaces of the materials are cleaned and then immediately passed through a rolling mill. The large plastic deformation causes the oxide layer to fracture and materials are able to bond with the aluminum base metal by heat and pressure. In the diffusion bonding method, the surfaces of the materials are cleaned and then pressed together using a mechanical force or by an isostatic pressure. The variables deciding whether there is a successful bond are the degree of the elimination of the oxide layer, temperature, pressure, and time. In the ultrasonic bonding method, ultrasonic energy and pressure induce an oscillating shear force that fractures the oxide layer and then produces a metallurgical bond. In the explosive welding method, the materials are placed at an angle and a detonator causes one plate to impact the other plate. The force of the impact removes a thin layer from the material surfaces and the high pressure causes the materials to bond. Bond quality is dependent on collision angle, impact velocity, material properties, and geometry. The friction welding method uses a rotary or linear motion of two materials against each other. An advantage of friction welding, like explosion welding, is that the welding process itself removes the oxide layer.
There are many types of material that can be bonded. However, achieving a successful bond of aluminum and copper is difficult. The materials react and form intermetallics above 120° C. Intermetallic Al2Cu forms first, followed by Al4Cu9, and then AlCu. In the present application, the diffusion bonding of aluminum alloys and copper alloys is described.
There are two basic types of diffusion bonding. One is to use a mechanical force to apply pressure to bond the components, and the other is to use a pressure chamber to apply isostatic pressure. The method of applying a high pressure gas force is known as Hot Isostatic Pressing (HIP). Both methods can be used to stack components so that multiple components can be bonded in a single batch process.
Within the domain of diffusion bonding there are subsets of the two basic processes. Shirzadi (1997) presents an analysis of six Transient Liquid Phase methods for diffusion bonding AlSiC to AlSiC (Aluminum Silicon Carbide). However, Transient Liquid Phase methods are costly and not well suited for high volume production.
A few patents teach methods of interlocking aluminum and copper to form a stronger bonded material that provides better thermal and electrical conductivity. U.S. Pat. No. 4,015,099 to Seniuk and Gagnon (1977) discloses a method to silver coat a threaded copper button, and after threading into an aluminum piece, preheating the assembly to between 190° C. and 245° C., and then arc welding using aluminum filler under an inert gas shield. U.S. patent application 2014/0017512 to Iimori and Hopper (2014) teaches the use of a copper-plated aluminum button which passes through a cloth member, and plastically deforms to interlock with a concave mating flange. U.S. provisional patent application 62/097,605 to Remsburg (2014) teaches a single interlocking layer between closely matched materials.
However, the above disclosures all fail to provide an effective interlock mechanism to increase the strength, electrical conductivity, thermal conductivity, and CTE matching of the stacked dissimilar metals.